1. Field of the Invention
This invention relates to measuring instruments and, more particularly, to instruments for testing circuit resistance and providing an audible tone upon the detection of resistance below a selected value.
2. Description of the Prior Art
Audible test devices for use in testing electrical or electronic equipment for electrical continuity and for measuring resistance have long been available in the prior art. For example U.S. Pat. No. 3,284,707 discloses a continuity or resistance testing device having an audible output, the tone of which varies with the test resistance. Circuit parameters are preselected such that the highest audio tone will be produced if the value of the resistance under test is zero and the lowest audio tone will be produced if the value of the resistance under test reaches infinity (open circuit condition). Other values of the test resistance between the two extreme values will cause audio tones to be produced intermediate the two limiting rates and, as a result, the patent disclosure indicates that a user would be able to distinguish between the values of different test resistances by the frequency of the audible tone produced.
U.S. Pat. No. 3,758,855 discloses a resistance controllable indicator which includes a frequency generating circuit, the frequency of which is inversely proportional to the value of the test resistance. A speaker may be used to indicate resistance values, the circuit component values being adjusted to provide a frequency response within the audible range.
Test circuits for indicating distinct voltage levels by audible means have also been developed. For example, U.S. Pat. No. 3,903,471 discloses an electronic test circuit that produces several audio tones that correspond to one of several digital logic voltage levels which allows the testing of microelectronic equipment audibly rather than visually, as with an oscilliscope or voltmeter. When the circuit under test is at a high voltage level, an oscillator is energized so that the sound signal with a predetermined frequency may be heard through a transducer. If a low voltage level is detected, a sound signal with a frequency different from that of the sound indicating the high voltage level will be produced.
Although U.S. Pat. Nos. 3,284,707, 3,689,832, and 3,758,855 disclose circuits which generate an audio tone that varies with measured resistance, the resistance sensitivity of each is such that the frequency produced within the range of from a few milli-ohms to a few ohms would hardly be noticeable. Although these references indicate that operable ranges from 0 to 1000 ohms can be achieved, if the user is concerned about resistance values of, for example, 1/100 ohm, 1/3 ohm or 1 ohm, these devices would be inadequate. It would also be extremely difficult for a user to audibly distinguish between slight variations in frequency over a comparatively large resistance range in order to accurately determine the test resistance value.